The purpose of this project is to understand cytokine regulation in animals infected with Histoplasma Capsulatum. Histoplasmosis is a disease found in certain geographic regions of this country. In a normal host the disease will often be controlled by an effective cellular immune response. By contrast, patients infected with HIV have a difficulty in eradicating the disease. Therefore it was of interest to study mechanisms which might alter the immune response to this organism with the goal of protecting the animal. Previous work had demonstrated that the organism resides in mononuclear phagocytic cells. Moreover, it was shown that the presence of IFNgamma enables the macrophage to kill the organism. Recently, a new cytokine, IL-12 has been shown to be a potent inducer of IFNgamma from both T cells and NK cells. Thus we set out to investigate the role that IL-12 had in the course of the disease and how it affected cytokine production especially IFNgamma. Mice infected with H. capsulatum and treated with neutralizing antibodies to IFNgamma, TNFalpha or IL-12 had accelerated mortality, indicating that endogenous production of these cytokines plays an important role in response to infection. In contrast, mice treated with IL-12 were developed a protective immune response. The protective effect of IL-12 could be abrogated if a neutralizing antibody to IFNg was given at the same time, demonstrating that the role of IL-12 in protection was mediated by IFNgamma. The above observations suggest that IL-12 may be useful in immunologic intervention against this opportunistic pathogen. The next study we performed was whether IL-12 was effective in immunodeficient animals infected with H. capsulatum. In this study, we examined the effect of using both IL-12 and Amphotericin B (AmB) as a treatment for immunodeficient SCID mice infected with H. capsulatum. AmB was added to see if there was an additive or synergistic effect of using IL-12 with a specific drug. Moreover, AmB is the drug used as initial therapy in patients with disseminated histoplasmosis. SCID mice infected with H. capsulatum and treated with IL-12 showed an increase in survival and a reduction in the colony counts of H. capsulatum at 14 days after infection. The effect of IL-12 was abrogated if animals were also treated with a neutralizing antibody to IFNgamma, IL-12 treatment also resulted in an increase in mRNA expression and protein production for IFNgamma, TNFalpha and nitric oxide from spleen cells. When IL-12 was combined with AmB treatment, there was a significant increase in survival compared to either modality alone. Moreover, combined treatment resulted in an increase in both IFNgamma and TNFalpha production as well as a substantial reduction in H. capsulatum burden at 35 days post infection. This study suggests that IL-12 and AmB can be used together as a treatment for H. capsulatum in immunodeficient hosts.